The ${\rm B_c^+(2S)}$ to ${\rm B_c^+}$ and ${\rm B_c^{*+}(2S)}$ to ${\rm B_c^+}$ cross section ratios, as well as the ${\rm B_c^{*+}(2S)}$ to ${\rm B_c^+(2S)}$ cross section ratio, are measured in proton-proton collisions at $\sqrt{s} = 13$ TeV, using a data sample collected by the CMS experiment, corresponding to an integrated luminosity of 143 fb$^{-1}$. The three measurements are made in the ${\rm B_c^+}$ phase space window defined by transverse momentum $p_{\rm T} > 15$ GeV and absolute rapidity $|y| < 2.4$, with the excited ${\rm B_c^{(*)+}(2S)}$ states reconstructed in their ${\rm B_c^+} \pi^+ \pi^-$ decay. The ${\rm B_c^+(2S)}$ to ${\rm B_c^+}$, ${\rm B_c^{*+}(2S)}$ to ${\rm B_c^+}$, and ${\rm B_c^{*+}(2S)}$ to ${\rm B_c^+(2S)}$ cross section ratios, including the (unknown) ${\rm B_c^{(*)+}(2S) \to B_c^+}\pi^+\pi^-$ branching fractions, are ${\rm (3.57 \pm 0.69~(stat) \pm 0.32~(syst))\,\%}$, ${\rm (4.91 \pm 0.69~(stat) \pm 0.57~(syst))\,\%}$, and ${\rm 1.39 \pm 0.35~(stat) \pm 0.09~(syst)}$, respectively, where the first uncertainty is statistical and the second is systematic. None of these ratios shows a significant dependence on the $p_{\rm T}$ or $|y|$ of the ${\rm B_c^+}$. The shape of the invariant mass distribution of the dipions emitted in the ${\rm B_c^{(*)+}(2S) \to B_c^+}\pi^+\pi^-$ decays is also reported.
Relative cross sections of the Bc(2S) and Bc(2S)* states with respect to the Bc.
Invariant mass distribution of the dipion system.
Invariant mass distribution of the dipion system.
Signals consistent with the B$^+_\mathrm{c}$(2S) and B$^{*+}_\mathrm{c}$(2S) states are observed in proton-proton collisions at $\sqrt{s} =$ 13 TeV, in an event sample corresponding to an integrated luminosity of 140 fb$^{-1}$, collected by the CMS experiment during the 2016, 2017, and 2018 LHC running periods. These excited $\bar{\mathrm{b}}$c states are observed in the B$^+_\mathrm{c}\pi^+\pi^-$ invariant mass spectrum, with the ground state B$^+_\mathrm{c}$ reconstructed through its decay to J/$\psi\,\pi^+$. The two states are well resolved from each other and are observed with a significance exceeding five standard deviations. The mass of the B$^+_\mathrm{c}$(2S) meson is measured to be 6871.0 $\pm$ 1.2 (stat) $\pm$ 0.8 (syst) $\pm$ 0.8 (B$^+_\mathrm{c}$) MeV, where the last term corresponds to the uncertainty in the world-average B$^+_\mathrm{c}$ mass.
Observation of the Bc(2S) and Bc(2S)* states and measurement of the Bc(2S) mass.
Observation of the Bc(2S) and Bc(2S)* states and measurement of the Bc(2S) mass.
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